The detection and debugging of error conditions is an important aspect of computer program execution. In some examples, program developers may be customers of a computing service provider that hosts execution of a program on behalf of the developer. This arrangement may often provide a great convenience to developers, for example by allowing them to deploy and execute their programs without having to build, maintain and manage their own program execution infrastructure. Computing service providers and other program execution infrastructures may sometimes allow execution of large scale program fleets including large quantities of program instances.